Technical Field
The present disclosure relates to a light emitting device and the manufacturing method thereof, in particular to a chip-scale packaging light emitting device including a light emitting diode (LED) semiconductor die which generates electromagnetic radiation while it is in operation.
Description of the Related Art
In the past decades, LEDs are widely used in applications including signal lamps, backlight units, general lightings, mobile components, automotive lightings and so forth. Generally an LED semiconductor die is disposed within a package structure, and/or covered by a photoluminescent material along a light irradiation path, to form a light emitting device.
Good light extraction efficiency of a light emitting device and a desired viewing angle mandated by applications can both be achieved through a proper geometric design of a package structure. For example, the traditional Plastic Leaded Chip Carrier (PLCC) LED package is widely used as an economical package structure to form white LEDs. For PLCC-type package structure, light extraction efficiency as well as the desired viewing angle can be optimized effectively through a proper design of the shape of a reflective cup surface. However, there are intrinsic limitations in PLCC-type LEDs, including: 1) Poor spatial color uniformity, due to large variations of the lengths along various optical paths of different light emitting angles, which will result in a projected yellow ring or a blue ring; 2) The light emitting surface area of a PLCC-type phosphor-converted light emitting device is significantly larger than that of a LED semiconductor die that is bonded inside the package. A larger light emitting surface area will result in larger etendue of the LED as a light source and complicates the design of a secondary optical lens accordingly; and 3) PLCC-type LED package has poor heat dissipation due to relatively higher thermal resistance between the LED semiconductor die and a lead frame.
Recently, another approach to fabricate much smaller form-factor phosphor-converted LEDs using flip-chip LED semiconductor dies, the so-called chip-scale packaging (CSP), can effectively address those problems commonly found in PLCC-type LEDs. Advantages of CSP-type LEDs include, but are not limited to, an approximately ideal point light source because of a chip-scale compact light emitting surface area, and excellent heat dissipation path due to a flip-chip design. Furthermore, its simplified manufacturing process lowers manufacturing costs to fabricate CSP-type LEDs compared to PLCC-type LEDs.
However, some of the good optical properties commonly found in conventional PLCC-type LED packages are lost for CSP-type LEDs due to continuous shrinkage of the package size. For example, for those CSP-type LEDs, four edge surfaces of a photoluminescent structure, which are surrounded by reflectors on the four surfaces, are four vertically flat surfaces perpendicular to the light emitting (top) surface due to limitations of existing manufacturing processes. One drawback of the vertically flat reflector design is that most of the light inside the photoluminescent structure travelling toward the vertically flat side-reflector will be reflected back and trapped within the photoluminescent structure, and, even worse, be reflected into and absorbed by the LED semiconductor die, due to total internal reflection occurring at the top surface of the photoluminescent structure. This kind of package structure will reduce package extraction efficiency because more light will not be able to escape from the photoluminescent structure after wavelength conversion, resulting in a significant amount of optical energy loss inside the package structure of a light emitting device. Besides, there is still no proper solution currently provided to change the viewing angle of CSP-type LEDs.
Therefore, there is a need to design an improved CSP-type LED structure to further increase light extraction efficiency, improve the spatial color uniformity, reduce the radiation area to approximately an ideal point source, lower thermal resistance, and make the viewing angle tunable.